Modifying the Electrocatalytic Selectivity of Oxidation Reactions with Ionic Liquids

Abstract The “solid catalyst with ionic liquid layer” (SCILL) is an extremely successful new concept in heterogeneous catalysis. The idea is to boost the selectivity of a catalyst by its modification with an ionic liquid (IL). Here, we show that it is possible to use the same concept in electrocatalysis for the selective transformation of organic compounds. We scrutinize the electrooxidation of 2,3‐butanediol, a reaction which yields two products, singly oxidized acetoin and doubly oxidized diacetyl. When adding the IL (1‐ethyl‐3‐methyl‐imidazolium trifluormethanesulfonate, [C2C1Im][OTf]), the selectivity for acetoin increases drastically. By in situ spectroscopy, we analyze the underlying mechanism: Specific adsorption of the IL anions suppresses the activation of water for the second oxidation step and, thus, enhances the selectivity for acetoin. Our study demonstrates the great potential of this approach for selective transformation of organic compounds.


Adsorption of the Ionic Liquid on Pt(111)
By using subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS), we investigate the adsorption behavior of 0.05 M [C2C1Im] [OTf] in 0.1 M perchloric acid. Note that the acid was used as the supporting electrolyte in the butanediol oxidation experiment. For the SNIFTIRS experiment, we recorded reference spectra by using both s-and ppolarized radiation at 0.05 VRHE, then the potential was stepped to a higher potential where the sample spectra were collected in both s-and p-polarization, see Figure S1. During the experiment, the potential of the Pt(111) working electrode switched between 0.05 VRHE and the selected sample potentials. To correlate with the 2,3-butanediol oxidation experiment, we increase the sample potential from 0.1 to 1.1 VRHE with a step of 0.1 V. The spectra at different sample potentials were calculated as (Rsample-Rreference)/Rreference where the Rreference is the preceding spectra at 0.05 VRHE.
As shown in Figure S2, three negative bands appear in the frequency region between 1350 and 1150 cm -1 starting from 0.2 VRHE, which are the characteristic IR bands for [OTf]anion of the IL according to the literature. 1 Note that negative (pointing downwards) and positive (pointing upwards) bands indicate formed and consumed species, respectively. The peak at 1260 cm -1 is attributed to the asymmetric stretching vibration of the SO3group. The peaks at 1228 and 1183 cm -1 are ascribed as the symmetric and asymmetric stretching vibration of the CF3 group, respectively. A direct comparison of the spectra measured in p-and in s-polarization show that the bands are observable in p-polarization only. Note that EC-IRRAS bands observable in ppolarization only belong to adsorbed species due to the metal surface selection rule. [1] This confirms that the [OTf]anion adsorbs specifically and potential dependent on the Pt(111) surface in the potential region of 2,3-butanediol oxidation. Note that we expect similar behavior also for other [OTf]containing salts. Figure S1. The procedure used in the SNIFTIRS experiment shown in Figure S2. Figure S2. IR spectra measured in a SNIFTIRS experiment in both s-and p-polarization. The sample potentials increased from 0.1 to 1.1 VRHE with a step of 0.1 V. Before each sample potential, the potential was stepped to 0.05 VRHE, while the reference spectra were taken (see Figure S1).

The IR spectra of 2,3-butanediol, acetoin, and diacetyl
To assign the IR bands of 2,3-butanediol, acetoin, and diacetyl to the different vibrational modes we compared the experimentally obtained IR spectra using attenuated total reflection (ATR) IR spectroscopy with calculated IR spectra obtained by density functional theory (DFT) (see Figure  S3 and S5). We summarized the band assignment for 2,3-butanediol, acetoin, and diacetyl in Table  S1, S2, and S3, respectively. We visualized the different vibrational modes using the program QVibePlot [2] in Figure S4, S5, and S7. Figure S3: Infrared spectra of 2,3-butanediol in the spectral region from 2500 to 900 cm −1 . Simulated spectra of the meso and (S,S) isomers from DFT (black) and experimental IR spectrum of the racemate (orange) recorded with ATR IR spectroscopy. The band assignment is summarized in Table S1 and the vibrational modes visualized in Figure S4 and S5.  Figure S4: Visualization of the different vibrational modes and the corresponding band positions of the calculated spectra of meso 2,3-butanediol depicted in Figure S3. Figure S5: Visualization of the different vibrational modes and the corresponding band positions of the calculated spectra of (S,S)-2,3-butanediol depicted in Figure S3. Figure S6: Infrared spectra of the products acetoin and diacetyl in the spectral region from 2500 to 900 cm −1 . Simulated spectra from DFT (black) and experimental IR spectrum (orange) recorded in ATR geometry. The band assignment is summarized in Table S2 (acetoin) and Table S3 (diacetyl) and the vibrational modes visualized in Figure S7.    Figure S8 shows the EC-IRRA spectra of the 2,3-butanediol oxidation measured with p-polarized light. Note that species in solution as well as species adsorbed on the surface are detectable in these spectra. [1] At 0.3 VRHE, we observe a positive band at 1456 cm -1 and negative bands at 1713, 1359, and 1200 cm -1 . The band at 1200 cm -1 forms a maximum at 0.6 VRHE and decreases drastically at higher potentials. At 0.7 VRHE and above a further weak band appears at 2343 cm -1 . The positive band at 1457 cm -1 we assign to the δ(CH3) of consumed 2,3-butanediol. The negative bands at 1713, 1359 cm -1 are referred to the ν(CO) and ν(CC) vibrations of acetoin and diacetyl. As the band at 1200 cm -1 is a specific marker for the intermediate acetoin, we conclude that acetoin is initially formed as an intermediate with an onset potential at 0.3 VRHE and reaches highest concentration at 0.6 VRHE, while at higher potentials acetoin is further oxidized to diacetyl and the concentration decreases rapidly. The results measured with p-polarized light confirm the results measured with s-polarized light (Figure 2, main text). Finally, no adsorbed COads is observable in the spectral region between 1800 and 2100 cm -1 . This indicates that no CC bond splitting occurs on the Pt(111) surface and no COads is formed during the reaction. A similar behavior we observed also for other secondary alcohols as isopropanol. [3]

Quantification of IR bands
We analyzed the spectra shown in Figure 2 (main text) quantitatively using the approach described in Section 5 of the Supporting Information. Figure S9a shows the potential dependent concentration of 2,3-butanediol (green), acetoin (red), diacetyl (blue) as well as the sum of the products acetoin and diacety (grey) in the absence of the IL [C2C1Im] [OTf]. The onset of the 2,3butanediol oxidation occurs at 0.3 VRHE forming initially acetoin. The onset potential of diacetyl formation is 0.5 VRHE. The concentration of acetoin reaches a maximum at 0.6 VRHE and decreases at higher potentials rapidly, indicating the oxidation of acetoin forming diacetyl. At potentials ≥0.8 VRHE the 2,3-butanediol oxidation stops due the formation of a passivating hydroxide layer. Note that we assign changes in the concentration at potentials above 0.8 VRHE to diffusion between the thin layer and the bulk solution. In the presence of the IL (Figure S9b) only a small fraction of diacetyl is formed and the oxidation of acetoin is sufficiently suppressed.

Description of the quantitative analysis a) Determination of the extinction coefficients
To determination the extinction coefficients, a dilution series of 2,3-butanediol, acetoin, and diacetyl with different concentrations c was prepared, and the absorbance A was measured using a transmission cell with a layer thickness d of 50 μm (see Figure S10). The extinction coefficients  were determined from linear fits of the Beer Lambert law (see Figure S11): The obtained extinction coefficients are given in Table S4.

b) Determination of the average beam path in the thin film
We determined the average beam path of the IR beam in the thin layer using the Beer Lambert law. The absorbance was determined from the single channel spectra of 2,3-butanediol at 0.05 VRHE. Using the absorbance of the IR band of 2,3-butanediol at 1380 cm −1 and the extinction coefficient given in Table S4, we calculated the average beam path d of the IR beam in the thin layer:

c) Calculation of the concentration change of 2,3-butanediol, acetoin, and diacetyl
Using the determined thin layer thickness and the extinction coefficients of selected IR bands, we calculated the change of concentration of 2,3-butanediol, acetoin, and diacetyl as a function of potential.
First, we calculated the change of concentration of acetoin using the IR band of acetoin at 1200 cm −1 , which does not overlap with bands of 2,3-butanediol and diacetyl. The resulting data are displayed in Figure S12. To determine the change of concentration of diacetyl, we use the difference in absorbance ΔA between 1358 and 1337 cm −1 . Measuring the difference enables us to be independent of the background. However, ΔA of diacetyl (ΔAdiacetyl_1358) overlaps with ΔA of acetoin (ΔAacetoin_1358) (see Figure S13). Therefore, we calculated ΔAdiacetyl_1358 by subtracting ΔAdiacetyl_1358 from total ΔA (ΔAtotal_1358): Note that the difference in the extinction coefficients Δε1358 of 2,3-butanediol between 1358 and 1337 cm -1 is very small. Consequently, we did not take into account changes in the c2,3-butanediol for the determination of cdiacetyl.
Using the Beer Lambert law, we obtained: Using the determined thin layer thickness and the extinction coefficients, we calculated the concentration change of diacetyl. The result is shown in Figure S14.  To determine the change of concentration of 2,3-butanediol, we use ΔA of 2,3 butanediol (ΔAbutanediol_1380) between 1380 and 1337 cm -1 . However, ΔAbutanediol_1380 overlaps with ΔA of acetoin and diacetyl (see Figure S15). Therefore, we calculated ΔAbutanediol_1380 from ΔAtotal_1380, ΔAacetoin_1380, and ΔAdiacetyl_1380 : Using the Beer Lambert law, we obtained: Using the determined thin layer thickness and the extinction coefficients, we calculated the concentration change of 2,3-butanediol. The result is shown in Figure S16.
r Figure S15: Transmission IR spectra of 2,3-butanediol (blue curve), acetoin (orange curve), and diacetyl (green curve) with a layer thickness of 50 μm, and the illustration of ΔA1380 of 2,3butanediol, acetoin, and diacetyl. The selectivity of acetoin was determined by the following equation: The calculated selectivity of acetoin in both experiments is shown in Figure 3 in the main text.

Experimental part
Cleaning procedure To avoid contaminations from the equipment, we cleaned all glass and Teflon parts as well as all noble metal wires with a procedure described in the following. We stored all parts in a solution of NoChromix (Sigma Aldrich) and H2SO4 (Merck, EMSURE, 98%) overnight. Afterwards, we rinsed the equipment at least five times with ultra-pure water (Milli-Q Synergy UV, 12.2 MΩ·cm at 25 °C, TOC < 5 ppm) and performed 3 boiling and rinsing cycles in ultra-pure water. We annealed all noble metal wires in the flame of a Bunsen burner and rinsed the wires subsequently with ultra-pure water.

Preparation of Pt(111) and solutions
We annealed Pt (111)  [OTf] was synthesized and purified in-house following a procedure described previously. [4] We degassed all solutions by purging with Ar (Linde, 5.0) for at least 20 min.

EC-IRRAS measurements
To measure electrochemical infrared reflection absorption spectroscopy (EC-IRRAS) we used commercial IR spectrometers with evacuated optics (Bruker Vertex 80v) and liquid nitrogen cooled mercury cadmium telluride (MCT) narrow band detectors. The spectrometers are equipped with commercial optics for electrochemical measurements, home-build electrochemical cells and polarizers. We used CaF2 hemispheres (Korth) as IR transparent window material sealed by Kalrez gaskets. We controlled the potential by commercial potentiostats (Gamry Reference 600 or Reference 600+). We used Pt wires as counter electrodes and home build or commercial (Mini-HydroFlex, Gaskatel ) reversible hydrogen electrodes (RHE) as reference electrodes. We recorded reference spectra (256 scans per spectrum) at 0.05 VRHE before each measurement. We took IR spectra with 128 scans per spectrum, a resolution of 2 cm -1 , and an acquisition time of 57 s per spectrum between 0.05 and 1.1 VRHE. The spectra were measured with p-and s-polarized light, respectively. For a more detailed description of the used IR setups, we refer to literature. [5] Density functional theory We performed gas-phase density functional theory (DFT) calculations with the Turbomole Software package version 7.2. [6] The exchange-correlation functional PBE by Perdew, Burke and Ernzerhof [7] was used with the def2-TZVP basis set by Weigend and Ahlrichs [8] . The RI-J approximation [8b] was applied to accelerate the calculations. Long range dispersion interactions were included by using the D3 dispersion correction scheme. [9] The COSMO solvation model [10] was applied, using a dielectric constant of 80. Harmonic frequency calculations were performed with analytical gradients to obtain the vibrational spectra.